Non-linear optical properties and structure of Na2SGeS2 glasses

1o tYR NA I, o r

ELSEVIER

Journal of Non-Crystalline Solids 215 (1997) 61-67

Non-linear optical properties and structure of Na2S-GeS 2 glasses Zhong Hua Zhou, Hiroyuki Nasu *, Tadanori Hashimoto, Kanichi Kamiya Department of Chemistryfor Materials, Facul~ of Engineering, Mie University, Kamihama, Tsu, Mie 514, Japan Received 12 September 1996; revised 20 November 1996

Abstract

Third-order non-linear optical properties of Na2S-GeS 2 glasses were measured by the third harmonic generation method. T h e X(3) of the glasses was of the order of 10-12 esu, which is one order of magnitude larger than that of Na20-GeO z glasses. Unlike Na20-GeO 2 oxide glasses, the density, refractive index and X(3) value decrease upon addition of Na2S to G e S 2 glass, which suggests no structural change from the [GeS4] unit to the [GeS6] unit. Structural studies by IR and XPS methods revealed the formation of non-bridging sulfur in the glasses. It was found that the contribution of non-bridging sulfurs to X(3) is larger than that of bridging sulfurs.

I. Introduction

It has been reported that sulfide glasses show higher non-linear optical properties than oxide glasses and are expected to be suitable for photonics devices [1-3]. Comparing to oxygens in oxide glasses, sulfurs with a large atomic (ionic) radius may play an important role in the non-linear optical properties of sulfide glasses [2]. In oxide glasses, it has been found that the optical non-linearity depends on the type of chemical bonding of oxygens (bridging or non-bridging), e.g., the contribution of non-bridging oxygens to third-order optical non-linear susceptibility X (3) is larger than that of bridging oxygens in the alkali silicate glasses [4]. Furthermore, it has been reported that the coordination state of cations in the oxide glasses also

* Corresponding author. Tel.: + 81-592 31 9435; fax: + 81-592 31 9435; e-mail: [email protected]

affects non-linear optical properties. In N a 2 0 - G e O z glasses which exhibit the germanate anomaly or the appearance of a maximum or minimum in the property-composition curves [5,6], X (3) was first increased with increasing Na20 content then decreased on further Na20 addition with a maximum at about 15 mol% Na20 [7]. This phenomenon was attributed to a change in the coordination state of Ge atoms from fourfold to sixfold on the Na20 addition, and a larger contribution of sixfold coordinated Ge atoms to X (3) than fourfold ones. The magnitude of nonlinear optical susceptibility of a [GeO 6] unit was four times larger than that of a [GeO4] unit. Measurements of third-order non-linear optical properties by the THG method and examination of glass structure by IR and XPS methods are reported for Na 2S-GeS 2 glasses. The influence of the substitution of sulfurs for oxygens, type of chemical bonding of sulfur (bridging and non-bridging) and coordination state of Ge atom in the glasses on X (3) is discussed, comparing with N a 2 0 - S i O 2 and N a 2 0 GeO 2 glasses.

0022-3093/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 0 2 9 - X

62

ZH. Zhou et al./ Journal of Non-Crystalline Solids 215 (1997) 61-67

2. Experimental procedure

I '-~uaan shifter( H 2 ) '

l'0641Jan

[

Q'sw'Nd:¥AGlaser trig.

2.1. Glass samples

] :

1.9~m pump

Analytical grade reagents of Na2S and GeS z were used as starting materials. Glass samples of compositions, x N a 2 S . ( 1 0 0 - x ) G e S 2 ( x = 10, 15, 20, 30, 40 in mol%), were obtained by melting in vitreous carbon crucibles with caps under a nitrogen atmosphere at 850-950°C inside a glovebox and quenching the melts by sandwiching between two steel plates, followed by annealing at 250°C for 1 h. To minimize volatilization loss of sulfur, we chose the melting time of 15-20 min. X-ray diffraction analysis for the samples obtained was carried out in order to confirm the glass formation. The samples were polished to optical grade for optical measurements. Density, IR and XPS spectra of all samples were measured, but only low NazS (x = 10, 15, 20) samples were used for optical measurements. 2.2. Density, refractive index and optical transmission spectra The density was measured by the Archimedes method using kerosene as an immersion liquid at room temperature (19°C). The refractive index was measured over a wide wavelength range from 500 to 1000 nm using an ellipsometers (Mizojiri Optical Works, Model DVA-36vw). The UV-visible-near infrared spectrum was measured in the wavelength region from 190 to 2000 nm using a spectrophotometer (Shimadzu Co., UV-3100).

Rotator

Fig. 1. Schematic illustration of the set-up for the THG measurement.

dicular to the 1900 nm incident beam in order to change the optical path length and obtain Maker fringe pattern. 2.4. IR and XPS measurements Infrared (IR) and X-ray photoelectron spectroscopy (XPS) spectra were recorded to investigate the structure of the glasses. IR spectrum was measured using a KBr method from 1000 to 220 cm-1 with an IR spectrometer (JASCO Co., FT/IR-550). XPS spectrum was recorded using an ESCA spectrometer (Shimadzu Co., electron spectrometer ESCA 750). Glass samples were fractured to obtain fresh surface immediately prior to measurement. The vacuum condition of detecting chamber was kept at 1 X 10 -7 Torr. The peak position of S2p was calibrated by C]s peak (284.6 eV) [8].

2.3. THG measurement

3. Results

The third harmonic generation (THG) of glasses was measured by Maker fringe method with a nonlinear optical measurement apparatus (Tokyo Instrument, Inc.) schematically shown in Fig. 1. The laser with a wavelength of 1900 nm and 10 ns duration was generated by exciting a high-pressure hydrogen cell with 1064 nm beam of a Qswitched Nd:YAG laser. To avoid damage to glass samples, the 1900 nm beam was attenuated by means of neutral density filters before the sample. The sample was mounted on a goniometer and rotated from + 45 ° to - 4 5 ° with respect to the axis perpen-

3.1. Linear refractive index Fig. 2 shows the wavelength dependence of linear refractive index of the glasses. The refractive index of Na2S-GeS 2 glasses decreases with increasing wavelength and NazS content. Fig. 3 represents the plot of 1 / ( n 2 - 1) vs. E 2 (E: photon energy). A linear relation exists between 1 / ( n 2 - 1) and E 2, therefore, the refractive index at 1900 nm is obtained by extrapolating the linear relationship as shown in Fig. 3. The refractive indices at 633 and 1900 nm are listed as n3~, and n~ in Table 1.

Z H. Zhou et al. / Journal of Non-Crystalline Solids 215 (1997) 61-67 2.2

'

i

,

i

,

r

'

I•

~

\"~.

i

,

x=lS

I "_ x=2o

.I

Table 1 Optical properties of xNa2S. (100- x)GeS2 glasses

r

I

Glass

n3,,

n,o

.i

T3~o To, /30~ Ic X(3) (%) (%) (a.u.) (IJ,m) (10-J2esu)

x=10 2.121 2.081 23.0 35.0 5.07 4.26 5.02 x=15 2.110 2.050 26.5 38.0 4.88 3.76 4.49 x=20 2.077 2.029 37.0 43.5 10.32 4.11 4.03

2J

2.0

63

I

~

I

500

i

600

I

I

i

700

i

800

[

i

900

Wavelength

I

1000

/nm

Fig. 2. Wavelength dependence of the refractive index of xNa2S( i 0 0 - x)GeS 2 glasses ( x = 10, 15, 20).

The X (3) is calculated by the following equation: lc s ~ I30, n`on3`o 3 3 X(3) (3)-,° T°isT3`o,s =Xs lc /3,o .~~ no~.~n3o.s 3 3 ' T/oT3~

3.2. Third harmonic generation SiO 2 glass was used as a reference in THG measurement. The THG intensity measured as a function of the incident angle for SiO z glass and x N a z S . ( 1 0 0 - x ) G e S 2 glasses (x = 10, 15, 20) is shown in Fig. 4 and Fig. 5, respectively. The coherence length (/c) was obtained from the fringe patterns as follows:

(l_sin2Oj/n2),/2

Ic= 2(j~_i )

-

(1 - sin2Oi/n~,) 1/2

'

(1)

where d, 0i and 0j are the sample thickness, the incident angle of the ith and jth fringes, respectively.

'

i

*

0.34 - ~0~ •

,

~

i

where 13,0 and T stand for the intensity of the third harmonic generation and transmittance, respectively, and subscript s means the reference silica glass. In the present study, the value of X~3) = 2.8 × 10 -14 esu was used for the reference silica glass. Other optical properties of silica glass are lc,s = 18.1 I~m, T3,o,~ = 92.8%, T`0.~= 95.0%, n3`0,~ = 1.457 and n,o.s = 1.440. The transmittance, coherence length and X (3) values of N a z S - G e S 2 glasses are listed in Table 1. The error in X (3) values was estimated about to be 5% which was mainly caused from the measurement of 13,o. The plots of X {3) vs. Na2S content, refractive index and sulfur ion number density in the glasses are shown in Fig. 6(a)-(c), respectively. The X (3) of the glasses is in the order of 10 -12 esu and decreases with increasing Na2S, as seen in Fig. 6(a).

3.3. IR spectra IR spectra of the glasses are shown in Fig. 7. The main absorption peak around 400 c m - l is attributed

x-=10 | •

0.32 - ~ =

i

•

"

x=15

I

X=20 I

0.30 0.28

0.26 I

2

,

i

4 E 2 /eV2

i

6

Fig. 3. Dependence of the refractive index on photon energy in

xNa2S .(100- x)GeS2 glasses (x = 10, 15, 20).

(2)

-40

-20 Incident

0 angle

20

40

/deg.

Fig. 4. Maker fringe pattern of the reference SiO 2 glass.

64

Z.H. Zhou et al. / Journal of Non-Crystalline Solids 215 (1997) 61-67 i

,

i

,

i

,

i

,

i

,

i

,

i

'1

(a) 10Na2S • 90GeS2 . . . . . . . . .

I

171- . . . . . . . . . . . . .

~,.

~

50em'l

i • '

'

'

1

'

I

'

I

'

'

'

I

"

t

i

I

,

I

'

I

'

1

'

i

'

,

X = 30 ~ . ~ . .

~

X = 20

il

4 0 0 e m "1

"~

(b) 15Na2S • 85GES2 J

I

1000 '~

¢~

-20

-10 0 10 Incident angle /deg.

20

Eb

to the asymmetrical stretching vibration of GeSbridging bonds in [GeS 4] tetrahedral units. The addition of Na2S to GeS 2 glass causes a new absorption band at about 450 cm- 1, intensity of which increases with the increase of Na2S content. This peak is attributed to the vibration o f Ge-Snon_bridging-bondsin [GeS 4] units [9].

S2p XPS spectra are shown in Fig. 8. It is evident that there is a significant change in both the position and shape of the peak with increasing Na 2S content.

10 - u

(3)

I

(b)

g

,

0

I

,

I

10 20 Na 2S tool%

,

I

1

(c)

2S

Na2S.

lOmol%"

20m01%

"12

I Na

~

10

= hu- E k

(hu: energy of X-ray), the binding energy (E b) of non-bridging sulfur is lower than that of bridging one. Therefore, the shift of the peak toward the lower binding energy side may be due to the formation of non-bridging sulfur. To examine the relative amount of Snb (non-bridging sulfur), the S2p XPS spectra obtained were smoothed and deconvoluted using a four-peak separation method which has been applied to TI2S-As2S 3 glasses by Heo et al. [10] and to T12S-GeS 2 glasses by Almeida et al. [11]. An example of peak separation is given in Fig. 9. The two peaks (1 and 2) at higher binding energy (163.4 and 162.1 eV), and the

S2p XPS spectra

(a)

i

This implies that the structure changes with adding Na2S to GeS 2 glass. The kinetic energy (E k) of photoelectrons of non-bridging sulfur is larger than that of bridging one and, according to the relationship

30

Fig. 5. Maker fringe patterns of x N a 2 S . ( 1 0 0 - x)GeS 2 glasses. (a) x = 10, (b) x = 15, (c) x = 20.

3.4.

I

Fig. 7. IR spectra of xNa2S . ( 1 0 0 - x)GeS 2 glasses (x = 10, 15, 20, 30, 40).

20Na2S • 80GeS2

-30

i

800 600 Wavenumber /cm"1400

lSmol%

I

I

10tool%" 15mo1% ~0mol%

I

I

,

I

,

2.0 2.1 2.40 2.45 2.50 2.55 S2" number Refractive index n3 density /1022ioncm -3

Fig. 6. (a) X O) vs. amount of Na2S in x N a 2 S . ( I 0 0 - x ) G e S 2 glasses ( x = 10, 15, 20), (b))((3) VS. refractive index n in xNa2S . ( 1 0 0 - x ) G e S 2 glasses ( x = 10, 15, 20). (c) X (3) vs. S 2- number density in xNa2S . ( 1 0 0 - x ) G e S 2 glasses ( x = 10, 15, 20).

Z.H. Zhou et al. / Journal of Non-Crystalline Solids 215 (1997) 61-67 i

,

65 i

•

i

!-- convolutedspectrum ~1 J P~k 1: S ~ " a [ rN peak2: S~2p~ l I ~,

I P'ka:S'b2, '~

_.!x= 3o

/

1 x

x = 20

x ~ 10 ~

170

I

~

165 Binding energy /eV

I

160 170

Fig. 8. S2p XPS spectra of x N a 2 S . ( 1 0 0 - x)GeS 2 glasses ( x = 10, 15, 20, 30).

Fig. 9. Four-peak deconvolution of the S2p XPS for 30Na2S. 70GeS 2 glass.

other two peaks (3 and 4) at lower binding energy (161.9 and 161.0 eV) are attributed to bridging sulfur and non-bridging sulfur, respectively. The details of the peak assignment is listed in the insertion in Fig. 9. Therefore, the relative amount of non-bridging sulfur can be computed from the combined area under the non-bridging sulfur peaks (peaks 3 and 4 in Fig. 9) divided by the total area of the S2p band. The relevant parameters used for the four-peak deconvolution procedure are listed in Table 2. The average binding energy split between bridging and non-bridging sulfur is 1.2 eV.

The X O) values of the Na2S-GeS 2 glasses are large in one order of magnitude, compared with those of N a 2 0 - G e O 2 glasses [7]. From Fig. 6(c), the X ~3) value decreases almost linearly with the decrease of sulfur ion number density. In the case of N a 2 0 - G e O 2 glasses the germanate anomaly has been observed for X (3) [7]. X °) increased with Na20 content up to about 15 mol%, then decreased on further addition of NazO. The increase of X °) was attributed to the increase of sixfold coordinated Ge atoms or [GeO6] units formed at the expense of [GeO4] units without forming non-bridging oxygens. On the contrary, in the present Na2S-GeS 2 glasses, the density, refractive index and X ~3) decrease monotonicly with increasing Na2S content. This may suggest that no [GeS 6] units are formed in the Na2S-GeS 2 glasses as predicted from the fact that the radius ratio 0.29 for Ge4+/S 2- is relatively far from the radius-ratio limit for sixfold coordination (>0.414), while the radius ratio 0.39 for Ge4+/O2- is close to the limiting value which

4. D i s c u s s i o n

The X ~3) value of the Na2S-GeS 2 glasses increases with the increase of refractive index as seen in Fig. 6(b), which is in agreement with the following semi-empirical Miller's rule [12]: X(3)= I rt2-

~

1 ]4

1 × 10-,0 (esu).

165 160 Binding energy /eV

(4)

Table 2 S2v XPS peak parameters for xNa2S • (100 - x ) G e S 2 glasses (B.E.: binding energy) Glass

x x x x x

= = = = =

10 15 20 30 40

Sb2pl/2 B.E. (eV)

Area(%)

Sb2p3/2 B.E. (eV)

Area(%)

Snb2pl/2 B.E. (eV)

Area(%)

Snb2p3/2 B.E. (eV)

Area(%)

Snb(%) Calc. Exp.

164.7 164.3 164.5 163.4 165.5

29.2 25.7 24.5 20.6 15.7

163.5 163.2 163.4 162.1 164.3

60.9 57.5 51.0 41.1 34.7

163.4 162.9 163.1 161.9 164.0

3.3 6.6 8.5 12.7 16.9

162.5 162.0 162.2 161.0 163.1

6.7 10.1 16.0 25.5 32.7

10.5 16.2 22.2 35.3 50.0

10.0 16.7 24.5 38.2 49.6

66

Z.H. Zhou et al. / Journal of Non-Crystalline Solids 215 (1997) 61-67

enables Ge 4+ ions to have sixfold coordination with respect to O 2- in some cases. Instead, the addition of NaeS to GeS z glass may cause the formation of non-bridging sulfur associated with [GeS 4] units, as seen from the appearance of 450 cm-1 peak in IR spectrum (Fig. 7). The present result is consistent with the previous ones obtained for GeSe-Ag 2S-AgI glasses using EXAFS method [13], and for G e - S - B r glasses by IR and Raman methods [9]. To confirm our result directly, the X-ray radial distribution analysis of the Na2S-GeS 2 glasses is now in progress. Eventually, larger X (3) of the present glasses than corresponding oxide glasses may not be due to sixfold coordinated Ge, but is attributed to the substitution of sulfurs for oxygens. As mentioned above, NaeS acts as a network modifier in the NaeS-GeS 2 glasses, and non-bridging sulfurs should be formed and increased with the increase of NaeS content. Then, we assumed that the basic structural unit in the Na2S-GeS e glasses is [GeS4] tetrahedron and that 1 mol of NaeS added to GeS 2 glass forms 2 mol of non-bridging sulfurs in a similar way of adding NaeO to SiO 2 glasses, in which basic structural unit is [SIO4]. Therefore, the relative amount of non-bridging sulfur could be calculated by the following equation: Snb%

=

2x 200 - x

-

the Na2S-GeS 2 glasses to third harmonic generation, we suppose that there are two types of ion pair, that is G e - S - G e and G e - S - N a +, in the glasses according to the above structural discussion. The dipole moment of each ion pair, p, can be expressed as [14] otO)(Eloc(t)) 3 6 '

p ( t ) = a~t)Eloc(t) +

where a °), a~3) and Ejoc are the polarizability, the hyperpolarizability and the local electrical field, respectively. The X ~3) is expressed as a function of O/(3) as X ( 3 ) ( - - 6 0 ; 601, ¢-O2, O93)

L(to) L(,o1) L(toe) L(to3) =

24

E N / o ~ 3) ,

where i is the type of ion pair, N~ is the number density of i ion pair and L(to) is related with the local electric field, which is expressed approximately using the Lorentz function as [14] L(to) = (n~ + 2 ) / 3 .

(8)

In the present THG method, tol = toe = to3 and to = 3to 1, that is,

100%,

(5)

X (3) = -~-~L(3 to) L ( t o ) 3 Z N / o / ~ 3 ) .

(9)

Considering n3~o is directly measurable and no, is close to n3o: the Eq. (9) is approximated to 1 (n~o~+2)

4

The number density of each ion pair was calculated from the composition, density of the glasses and the fraction of each ion pair derived from the Eq. (5). The hyperpolarizabilities of G e - S - G e and G e -

Table 3 Parameters used for calculating X (3) of xNaeS • (100 - x)GeS 2 glasses

x = 10 x = 15 x = 20

(7)

1 X

where x is the mol% of NaES (x = 0 to 40). The calculated Snb values are also listed in Table 2. Excellent agreement of the observed values of Snb with the calculated ones implies that the proposed structural model is reasonable. Therefore, it is proper to compare the contribution of non-bridging sulfurs to X (3) in Na2S-GeS 2 glasses with that of nonbridging oxygen in Na20-SiO 2 glasses. To discuss the contribution of structural units in

Glass

(6)

Density (gcm- 3)

Molar volume (cm 3 mol- I)

Number density Ge-S-Ge (1022 ioncm -3)

G e - S - Na + (1022 ioncm -3)

2.858 2.814 2.790

45.79 45.48 44.80

2.24 2.05 1.88

0.26 0.40 0.54

vO) ~exp. (10- J2 esu)

Xc(ale, (10-12 esu)

5.03 4.49 4.03

5.04 4.45 4.06

Z H. Zhou et al. / Journal of Non-Crystalline Solids 215 (1997) 61-67

S - N a + ion pairs estimated by the least square fitting method are 0((3) Ge - S - Ge = 25.3 X 10 -35 esu cm3/ion, O((3)Ge-S Na + = 16.3

X 10 -35 e s u cm3/ion.

The X (3) values calculated by using t~Ge-S_Ge _(3) and 0((3) ce-s-Na + are well agreed with the experimental ones, as shown in Table 3. It may be reasonably assumed that the contribution of bridging sulfur to X (3), 0((3) (Ge- S)b , would be half of t~Ge'W(3)_S_Geand using the value of "~Na(3)+ (0.6 × l0 -35 esu cm3/ion) derived in Ref. [4], the hyperpolarizabilities of bridging and non-bridging sulfur are the following: (3) - S)b 0((Ge

~

12.7 × 10 -35 e s u cm3/ion,

a(3) (Ge - S)nb = 15.7 × 10 -35 e s u cm3/ion. Therefore, the hyperpolarizability of sulfur anion is in the order of 10 -34 e s u cm3/ion which is in one order larger than that of oxygen anion ( ~ 10 -35 esu cm3/ion) in oxide glasses [4,7]. This fact means that the contribution of anion to X (3) becomes very noticeable when large sulfur ion is substituted for oxygen ion. Furthermore, the hyperpolarizability of non-bridging sulfur is about 1.2 times larger than that of bridging one. Our research group has pointed out that the hyperpolarizability of non-bridging oxygen is about 1.4 times higher than that of bridging oxygen in Na20-SiO 2 glass [4]. This can be explained as follows. The average binding energy split between bridging sulfur and non-bridging sulfur is 1.2 eV in XPS spectrum, but is smaller than that of 2.1 eV between bridging and non-bridging oxygen. This implies that the increasing degree of electron density around sulfur ion when bridging sulfur turns to non-bridging sulfur is small compared with oxygen ion in oxide glasses.

5. Conclusion (1) The X (3) of the glasses is in the order of 10-12 esu, which is one order larger than that of N a 2 0 - G e O 2 glasses. (2) No occurrence of the germanate anomaly sug-

67

gests that basic structural units of the glasses do not change from [GeS4] to [GeS6]. Also, the fraction of non-bridging sulfurs detected by means of IR and XPS measurements changes according to the following equation: Snbr~

~--"

2 x~ (200 - x) • 100%,

x = 0 to 40.

(3) Since the contribution of non-bridging sulfurs to X (3) is close to that of bridging ones, X (3) of the glasses is dominated by only the total number density of bridging and non-bridging sulfurs and decreases with increasing Na2S content (decreasing S 2- number density).

Acknowledgements The authors wish to thank to Professor T. Yoko of Institute for Chemical Research of Kyoto University for helping with X (3) and refractive index measurements.

References [1] H. Nasu, R. Kubodera, M. Kobayasi, M. Nakamura and K. Kamiya, J. Am. Ceram. Soc. 73 (1990) 1794. [2] E.M. Vogel, M.J. Weber and D.M. Krol, Phys. Chem. Glasses 32 (6) (1991) 231. [3] H. Nasu, Y. lbara and K. Kubodera, J. Non-Cryst. Solids 110 (1989) 229. [4] H. Nasu, O. Sugimoto, J. Matsuoka and K. Kamiya, J. Non-Cryst. Solids 182 (1995) 321. [5] K. Kamiya and S. Sakka, Phys. Chem. Glasses 20 (1979) 60. [6] S. Sakka and K. Kamiya, J. Non-Cryst. Solids 49 (1982) 103. [7] O. Sugimoto, H. Nasu, J. Matsuoka and K. Kamiya, J. Non-Cryst. Solids 161 (1993) 118. [8] P.W. Wang and L. Zhang, J. Non-Cryst. Solids 194 (1996) 129. [9] J. Heo and J.D. Mackenzie, J. Non-Cryst. Solids 113 (1989) 1. [10] J. Heo, J.S. Sanghera and J.D. Mackenzie, J. Non-Cryst. Solids 101 (1988) 23. [11] R.M. Almeida, H. Nasu, J. Heo and J.D. Mackenzie, J. Mat. Sci. Lett. 6 (1987) 701. [12] C.C. Wang, Phys. Rev. B2 (1970) 2045. [13] A. Ibanez, E. Philippot, S. Benazeth and H. Dexpert, J. Non-Cryst. Solids 127 (1991) 25. [14] N.F. Borrelli and D.W. Hall, in: Optical Properties of Glass, ed. D.R. Uhlmann and N.J. Kreidl (American Ceramic Society, Columbus, OH, 1991) p. 87.Z